July 19, 2010 -- Children conceived by in vitro fertilization, more commonly called IVF babies, have a slightly higher risk of developing childhood cancer than babies conceived naturally, according to a new Swedish study.

But the study's main author emphasizes that childhood cancer is relatively rare and that the increased risk is small to moderate, and probably not caused by the IVF procedure itself but perhaps linked to the infertility.

"There is an increased risk for cancer in children born via IVF, but it's rather small," researcher Bengt Kallen, MD, PhD, a retired professor of embryology and head of the Tornblad Institute, University of Lund, Lund, Germany, tells WebMD. "The estimate that we give is that the risk increases 40%, but the estimate has, of course, a degree of uncertainty."

The study is published in the journal Pediatrics.

IVF Babies and Cancer Risk: Study Details

Kallen and his colleagues evaluated 26,692 Swedish children born via IVF from 1982 to 2005 using the Swedish Cancer Registry and comparing the number of children who had cancer and were conceived via IVF with those not conceived by IVF.

The new study adds to data from a previous study by the same researchers, which included evaluations of nearly 17,000 children. That study found nearly the same risk, but in the current study, the link is stronger.

The researchers took into account variables such as maternal age, number of pregnancies, previous miscarriages, body mass index, and other factors. They found 53 cases of cancer in children born via IVF, while 38 would have been expected statistically in the population.

The cancers included leukemia, central nervous system cancers, eye cancer, other solid tumors, and a condition called Langerhans histiocytosis (a condition in which there is an excess of a type of white blood cell). Experts don't agree whether it is a true cancer, the researchers write. But even when they excluded the six cases of histiocytosis, the increased risk still held for IVF babies and cancers, although it went down to a 34% increased risk.

Overall, considering all the cancers found, the IVF babies were 1.4 times, or about 40%, more likely to have a cancer during the follow-up period, through 2006.

IVF Babies and Cancer Risk: Why?

The study found an association, not cause and effect. And Kallen says the increased cancer risk is probably not caused by the IVF procedure itself.

He suspects factors such as complications in the newborn, or something about the infertility itself.

Other studies have found children born via IVF have an increased risk for health complications early in life and for more birth defects, Kallen writes. Some recent research, however, has found that the risk of birth defects in IVF babies is not much different than those in the general population.

Of his finding, Kallen says, "This is just one further slight complication."

IVF Babies and Cancer Risk: Message for Parents

What's the message for those undergoing or considering IVF? "I think that any couple who considers IVF should know that there is a moderately increased risk for complications for their children-to-be," Kallen says.

It's also important, he says, to maintain perspective. "Most IVF pregnancies end with the birth of a living, normal child, and the risk should not be exaggerated."

He suspects the findings from the Swedish study would hold true for the U.S. population but can't say that with certainty.

IVF Experts Weigh In

The new study findings are looked on as important by two IVF experts who reviewed the study for WebMD.

Although previous research has suggested IVF is associated with an increased cancer risk for the babies, the new study is believed to be the first to show a scientifically strong link, says William Gibbons, president of the American Society for Reproductive Medicine and professor and director of the division of reproductive medicine at the Baylor College of Medicine in Houston.

"More [studies] will help validate," he says. Based on what researchers know now, he says, "we think the risk is small."

He agrees with Kallen that the increased cancer risk may be associated with the infertility, not the IVF itself. "Avoiding IVF may not make the risk go away," he tells WebMD.

"The bottom line is this: If there is a risk, it is a small risk," James Goldfarb, MD, director of infertility and IVF at the Cleveland Clinic and president of the Society for Assisted Reproductive Technologies.

Regarding the six IVF-conceived children with a diagnosis of Langerhans histiocytosis, he says the condition is "most definitely not a cancer." Still, the link held after excluding these six cases.

In IVF, he says, "the critical factor is the number of embryos transferred. If you look at IVF over the years, the vast majority of health issues is due to multiple pregnancies."

Under guidelines from the fertility specialty organizations, Goldfarb says, doctors generally are advised to transfer no more than two embryos in women under age 35. In older women, more could be transferred, taking into account the woman's likelihood of getting pregnant and other factors, he says.

SOURCES: Kallen, B. Pediatrics, August 2010; vol 126P: pp 270-276.

Bengt Kallen, MD, PhD, retired professor of embryology, head of the Tornblad Institute, University of Lund, Lund, Sweden.

William Gibbons, MD, president, American Society for Reproductive Medicine; professor and director, division of reproductive medicine, Baylor College of Medicine, Houston.

James Goldfarb, MD, director of infertility and IVF, Cleveland Clinic.